Considerable interest has been generated in recent years in the development of multi-element thermoluminescent dosimeters and the associated dose calculation algorithms, especially as applied to large scale routine personnel dosimetry. Many facilities in the United States must comply with accreditation programs such as the Department of Energy Laboratory Accreditation Program (DOELAP), or the National Voluntary Accreditation Program (NVLAP). Prior dosimeters and associated algorithms have experienced difficulties in meeting the criteria set for low energy photons or mixtures of low energy photons with beta particles and/or neutrons.
U.S. Pat. No. 5,065,031 discloses a multi-element TL dosimeter together with a dose calculation method which are designed to enable users to meet the ever growing demands of modern personnel dosimetry and also environmental monitoring. The therein described methodology provides for improved interpretation of dosimeter response in terms of risk related quantity, i.e., dose equivalent. The dosimeter and method are capable of identifying the mixture type in a variety of mixed fields and estimating the relative contribution of major components. The dosimeter and dose calculation method enable a personnel dosimetry system to accommodate a wide range of radiation types and energies.
More particularly, the dosimeter described in the aforesaid patent is composed of two parts, namely a TLD card and a holder. The TLD card includes multiple thermoluminescent (TL) elements and the holder includes associated radiation modifying filters. Four TL elements and associated filters function as a basis for shallow dose measurement, a basis for deep dose measurement, for eye dose measurement, and as an energy spectrometer for low level energy photons, respectively. The dosimeter may include one or more other TL elements and associated filters for other purposes such as neutron dose estimation. Element Correction Coefficients (ECCs) are generated to relate the TL efficiency of each TL element of an entire dosimeter population (field dosimeters) to the mean TL efficiency of a small subset of this population which is used only for calibration purposes (calibration dosimeters). When an ECC is applied to the response of each individual TL element of any dosimeter, its TL efficiency is virtually identical to the mean value of the calibration dosimeters group.
As disclosed in the aforesaid patent, dosimeter response can be used to identify the radiation field mixture composed of beta particles and/or photons and to determine the relative contribution of each component in the mixed radiation field. This is accomplished by the use of mixture identification formulas that are based on superposition of radiation fields and the algorithmization of the response correlation between two pairs of TL element response ratios. Mixture identification curves (characterizations) are obtained and these are very different for various radiation field mixtures and compositions, thereby enabling identification of mixture type and component contribution. In U.S. Pat. No. 5,004,921 the methodology of the aforesaid application is expanded by providing for accurate interpretation of the dosimeter reading in terms of risk related quantity for neutron fields and mixtures thereof with photon and/or beta fields.
The foregoing and other conventional techniques have in the past relied upon development of algorithms and empirical data for interpretation of dosimeter data. Hence, the accuracy of such systems is dependent on the validity of the algorithms being used.